Study on Shrinkage Characteristic of Briquette During the Pyrolysis Process

STUDY ON SHRINKAGE CHARACTERISTIC OF BRIQUETTE DURING THE PYROLYSIS PROCESS

Qi Wang, Yongfa Zhang, Yuqiong Zhao Key Laboratory of Coal Science and Technology of Shanxi Province and Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, China

Abstract This thesis regards large size briquette as the research object. The shrinkage characteristic of briquette and effects heating rate and reaction temperature on shrinkage during pyrolysis process was studied with the measure equipment of contraction and mass loss under atmospheric pressure. The results indicated that with the increase of carbonization temperature, the briquette volume shrinkage was increased gradually, while the reaction time was decreased. The increase of heating rate led to the increase of shrinkage rates and the decrease of final shrinkage. The pyrolysis process of briquette includes depolymerization stage, polycondensation stage and crack formation stage.

Key words: coal pyrolysis, briquette, shrinkage characteristic

1. Introduction The shrinkage characteristic of briquette during the pyrolysis process have an significant influence on the coal’s combustion, gasification kinetics, ash formation and coking pressure[1]. Study on expansion and shrinkage characteristics of briquette can provide reference and guidance for coal conversion and comprehensive utilization. A cylindrical briquette (φ8mm×20mm) was studied by Bei Kunlun[2], and a quartz tubular reactor had been used to study the shrinkage characteristic of char produced at different heating rate. The results show that the rate of lateral shrinkage increases with the increase of final heated temperature. The shrinkage is caused by free radical fragment orientated association and crystalline ordered array. An oven wall[3] (600mm×600mm×400mm) was used to study the relationship between coking pressure, lateral shrinkage and vertical shrinkage during carbonization. The study showed that it is not until the coking pressure peak ends in the later stage of the carbonization process that the lateral shrinkage starts to develop. It’s also shown that the coke cake starts to shrink vertically when the downward force becomes greater than the upward frictional force between the coke cake and the oven wall. The researches above both choose single coal particle or small size of briquette as the research object, which cannot describe the shrinkage characteristic of large size（volume≥100mm×100mm× 100mm） of briquette in the actual industrial production. In that case, this thesis regards large size briquette as the research object and the shrinkage characteristic and process of briquette during pyrolysis process was studied with the measurement equipment of contraction and mass loss under atmospheric pressure. 2. Experiment 2.1 Properties and preparation of the samples The experiment take mixed coal that made of A, B, C and the hydrophilic polymer binder HR in certain proportion as the raw material, whose particle size is 100 mesh. The coal quality analysis results in Tab. 1.

Table 1. Raw coal quality analysis results (wt-ad%)

Proximate analysis（wt-ad%） Sample M A V FC A 13.45 31.68 25.77 29.1 B 13.62 11.92 31.75 42.71 C 3.85 5.24 29.01 61.9 HR 0.96 13.46 27.49 58.09

Weigh 1100g mixed coal accurately, and pour 200g water in it for several times to make the coal sample, then put the sample into precise cube steel mould(100mm×100mm×100mm), exert force , and the large size of briquette obtained. 2.2 The experimental apparatus and procedures The volume and mass of briquette were measured by measure equipment of contraction and mass loss under atmospheric pressure，which can measure the change of volume and mass loss precisely at the same time. Heat the equipment to 450℃, 550℃, 650℃ and 850℃ separately, then put the large size of briquette in it ,measuring the change of volume and mass under constant temperature. End the experiment when the number becomes unchanged. Put the large size of briquette in to the measure instrument, heating at a constant rate of 2℃/min, 3℃/min and 4℃/min separately to 900℃, then keep the constant temperature for 0.5h, measuring the changing rules of briquette volume shrinkage varying with time under different heating rate. The experiments mentioned above were taken at nitrogen atmosphere. 3. Results and discussion 3.1 The effects of reaction temperature on shrinkage

1.00 450℃ 0.00 550℃ 0.96 650℃ -4 -1.10x10 0.92 850℃ -2.20x10-4 450℃ 0 0.88

550℃ V/V dv/dt -4 650℃ 0.84 -3.30x10 850℃ 0.80 -4.40x10-4 0.76 0 30 60 90 120 150 0 30 60 90 120 150 t/min t/min a b Figure 1. Variation curves of briquette volume with time a: volume shrinkage curve; b: shrinkage rate curve

Fig.1 shows the coal shrinkage curves at different reaction temperatures. It can be seen that with the increase of carbonization temperature, the briquette volume shrink increased gradually, while the rate of shrinkage decreased. The increase of temperature leads to the decrease of the time of reaction reach to equilibrium. This is due to the high reaction temperature lead to more absorbing heat per unit time, more activated molecules and faster reaction[4] . It’s shown in Fig. 1b that with the time went on, briquette shrinkage rate under different reaction temperatures are approximately the same after 30min. It can be assumed that the furnace body temperature is high during this period, the heat provided was far more than the required reaction heat, and making reactions with different temperature requires can be carried out at the same time. 2

3.1 The effects of heating rate on shrinkage

1.02 0.0 2℃/min 0.96 3℃/min -6.0x10-4 4℃/min ) 0.90 -1

min -3 • 0 -1.2x10

0.84 % ( V/V 0.78 -3 -1.8x10 2℃/min 0.72 (dV/dt)/ 3℃/min -3 ℃/ -2.4x10 4 min 0.66 0 200 400 600 800 1000 0 200 400 600 800 1000 ℃ ℃ Temperature/ Temperature/ a b Figure 2. Effect of heating rate on coal contraction a: volume shrinkage curve; b: shrinkage rate curve

We can see from Fig.2a that the coal volume decreased with the temperature increasing at different heating rates. 3 ℃ /min shrinkage curve lies above 2 ℃ /min shrinkage curve. This is because that coal pyrolysis is an endothermic reaction, and when the heating rate was too fast, the time of tar cracking was shortened. Parts of the structure could not release the cleavage products promptly, and lead to the thermal hysteresis [5-6]. But before 600 ℃℃ , 4 /min heating curve was between 2 ℃ /min and 3 ℃ /min curve. It is because that in the initial stage, the main reaction was the pyrolysis of coal macromolecular structure. The larger the heating rate, the more heat absorption per unit time and the faster reaction. It can also obtained from Fig. 2a that the lower the heating rate, the more the final shrinkage, that is because with the decrease of heating rate, the time reached to the same temperature was longer, which lead to more full reaction and larger shrinkage degree. It can be also obtained from Fig.2a that under different temperatures, the shrinkage degree difference in high temperature region (500 ℃ ~ 900 ℃ ) was obvious, while the difference was not affected in the low temperature region. This is because low heating rate has been able to meet the demand of the reaction. When the reaction reached into high-temperature region, the energy need of main reaction increased. Low heating rate cannot meet the needs of reaction, making the difference of reaction increases. 3.3 Analysis of briquette shrinkage process Put the large size of cold briquette into measure equipment and raise the temperature to 900 ℃ with the rate of 3 ℃ /min. The volume shrinkage curves and volume shrinkage rate curve have been shown in Fig. 3.

1.02 0.0 0.96 (dV/dt)/

0.90 -8.0x10-4 0 ( 0.84 % • min V/V -1.6x10-3

0.78 -1 )

0.72 -2.4x10-3

0 200 400 600 800 1000 Temperature/℃ Figure 3. The Briquette volume shrinkage and shrinkage rate curve of coal pyrolysis process 3

It can be seen from Fig.3 that according to volume shrinkage curves, coal pyrolysis process can be divided into three stages: depolymerization stage, polycondensation stage and Crack formation stage. The first stage is depolymerization stage(20℃～400℃). In this stage, the external water and adsorbed gas (CO2, CH4) of briquette were removed, which lasts shortly. Then the binder was melted and lead to the new arrangement of pulverized coal particles, Coal and binder started to go through thermal decomposition, whose reaction were splitting and pyrolysis of coal and bind polymerization of the briquette and binder. Two kinds of cracking reaction mainly occur in this stage. In the first kind of cracking reaction, the forked chain and bridge bond of coal macromolecular structure has been broken and turned to be free radicals and other small molecular compounds like CH4, H2. In the other kind of cracking reaction, hydrogen rearrangement or transfer occurred on the adhesive side of the binder molecules, and the binder molecules is translated into H2 and free radical molecules. The free radicals were taking polymerization reactions at the same time, combining different coal particles together by chemical bonds and carbon structures. The second stage is polycondensation stage (400℃-650℃).The briquette shrinks sharply in this stage, whose reaction were depolymerization and decomposition that includes bridge bond of large moleculars’ fracture to generate free radicals, the aliphatic side chains of aromatic nucleus’ fracture to format volatile gas and partly oxygen containing functional groups’ fracture. When the carbonization temperatures continue to rise to 600 ℃, liquid products of solid colloid gradually decomposed: Part of liquid products released as the decomposition products, the other part of liquid products polycondensated and consolidated with solid state free radical, and formed solid state briquette. In this stage, large amounts of gaseous substances continued to produce and the emission of tar is high. The third stage is crack formation stage (650℃-850℃). In this stage, the shrinkage was basically completed. polycondensation reaction of aromatic nucleusin is the main reaction of this stage but reacted very slow. The solid product’s carbon content is higher, the structure is compact and the density increases. The volume of briquette is still slowly reduced, and then appeared the crack, which was expanding gradually. Conclusions 1. As the result of heat conduct, with the increase of carbonization temperature, the briquette volume shrinkage was increased gradually, while the time of reaction reach to equilibrium was decreased. The shrinkage rate under different temperatures close to each other after reacted 30min. 2. The increase of heating rate leads to the increase of shrinkage rates, which will make the shrinkage curve move to the high-temperature zone and results the thermal hysteresis. The smaller heating rate leads to the larger final shrinkage. Under different temperatures, the shrinkage degree difference in high temperature region (500 ℃℃ ~ 900 ) was obvious, while the difference was not affected in the low temperature region. 3. The pyrolysis process of briquette includes depolymerization stage, polycondensation stage and crack formation stage. During the depolymerization stage the main reaction was the depolymerization reaction of coal and binder. At the same time there are also free radicals taking polymerization reactions. The main reaction of polycondensation stage was depolymerization and decomposition that includes bridge bond of large moleculars’ fracture, the aliphatic side chains of

4 aromatic nucleus’ fracture and partly oxygen containing functional groups’ fracture. The volume of briquette during crack formation stage is slowly reduced, and then appeared the crack.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 512741 47), National Science & Technology Pillar Program (GrantNo.2012BAA04B03) and Shanxi Provi ncial Natural Science Foundation (2010011014-1). References [1]Fu Zhixin, Guo Zhancheng. Expansion and contraction characteristics of coal pyrolysis process with image method of on-line measurement [J]. Journal of fuel chemistry, 2010.33(05) [2]Bei Kunlu, Wang Peng, Zhang Yongfa, Study on Coking and shrinkage characteristics of blended coal under temperature gradient [J]. Coal conversion, 2006(01) [3] S. Nomura, T. Arima,Coke shrinkage and coking pressure during carbonization in a coke oven [J]. Fuel, 2000（79） [4] M.J. Blesa, J.L. Miranda, M.T. Izquierdo,R. Moliner,Curing temperature effect on mechanical strength of smokeless fuel briquettes prepared with molasses [J]. Fuel, 2003(82) [5] Seungdo Kim,Jae K. Park. Characterization of thermal reaction by peak temperature and height of DTG curves [J]. Thermochimica Acta, 1995 (264) [6] Mustafa Versan Kok, Simultaneous thermogravimetry–calorimetry study on the combustion of coal samples: Effect of heating rate [J]. Energy Conversion and Management, 2012(53).